This application claims the benefit of Korean Application No. P2000-85419 filed on Dec. 29, 2000, which is hereby incorporated by reference.
1. Field of the Invention
The present invention relates to a thin film transistor (TFT), and more particularly, to a method for fabricating a polysilicon TFT. Although the present invention is suitable for a wide scope of applications, it is particularly suitable for improving electrical characteristics of a polysilicon layer.
2. Discussion of the Related Art
In a conventional process for forming a polysilicon layer, an intrinsic amorphous silicon layer is formed on an insulating substrate by using a plasma chemical vapor deposition (PCVD) method or a low pressure chemical vapor deposition (LPCVD) method. When the amorphous silicon layer has a thickness of about 500 xc3x85 (angstrom), it is recrystallized into a polysilicon layer by a crystallization method. The crystallization method is generally classified into a laser annealing method, a solid phase crystallization (SPC) method, and a metal induced crystallization (MIC) method.
In the laser annealing method, an insulating substrate having an amorphous silicon layer thereon is heated to a temperature of about 250xc2x0 C. (degrees celsius). Thereafter, an eximer laser beam is applied to the amorphous silicon layer to form a polysilicon layer. For the SPC method, a heat-treatment is applied to the amorphous silicon layer at a high temperature for a long time to form the polysilicon layer. For the MIC method, a metal layer is deposited on the amorphous silicon layer and the deposited metal layer is used as a crystallization seed. In case of the MIC method, a large sized glass substrate may be used as the insulating substrate.
The laser annealing method is a recently widely researched method in forming a polysilicon layer. In the laser annealing method, laser energy is provided for an amorphous silicon layer formed on the insulating substrate, thereby melting the amorphous silicon layer. Then, the melted amorphous silicon is cooled to form polysilicon.
In case of the SPC method, a buffer layer is formed on a quartz substrate that can stand at a temperature higher than 600xc2x0 C. (degrees celsius). The buffer layer serves to prevent spreading a contamination that results from the quartz substrate. Thereafter, the amorphous silicon is deposited on the buffer layer and is sufficiently heated in a furnace at a high temperature so as to form the polysilicon layer. However, because the SPC method is performed at the high temperature, it is difficult to acquire a desired polysilicon phase.
In the process of the SPC method, because polysilicon grains are developed without a continuous directionability, the polysilicon layer formed by the SPC method may have an irregular surface. For a thin film transistor, a gate insulating layer covers the polysilicon layer. Therefore, if the polysilicon layer has the irregular surface, the gate insulating layer is also formed with an irregular surface, thereby decreasing a breakdown voltage of the thin film transistor. Further, since size of the polysilicon grains is very irregular, electrical characteristics of a device adopting the polysilicon layer may be deteriorated in the SPC method. Furthermore, the quartz substrate used for the SPC method is very expensive. Thus, fabrication cost can be increased.
Unlike the SPC method that uses the expensive quartz substrate, the MIC method uses a relatively inexpensive glass substrate to form polysilicon. In case of applying the MIC method, however, metal impurities may remain in the polysilicon network, thereby deteriorating the quality of the polysilicon layer. Accordingly, other methods have been developed to improve the MIC method.
A field effect metal induced crystallization (FEMIC) method is an example of the improved MIC method. In the FEMIC method, after metal is deposited on a substrate, a high density direct current is applied to the metal to cause Joule heating. Because of the heated metal, an amorphous silicon formed on the heated substrate is crystallized into polysilicon. In this process, the metal serves as a catalyzer and is referred to as a catalytic metal.
FIGS. 1A to 1F illustrate a process of forming a polysilicon TFT according to the related art. The polysilicon TFT in this illustration is a coplanar type TFT having a top gate structure, and the FEMIC method is used to form the TFT.
In FIG. 1A, a first insulating layer 2 (i.e. a buffer layer) and an amorphous silicon layer 4 are sequentially deposited on a substrate 1. The buffer layer 2 is to protect the amorphous silicon layer 4 from alkali substances, which may be produced from the substrate 1 during later processes. Generally the buffer layer 2 is formed of silicon oxide (SiO2). After the amorphous silicon layer 4 is formed, thin catalytic metal clusters 8 are formed thereon. Nickel (Ni) is typically selected for the catalytic metal among a transition metal such as nickel (Ni), paladium (Pd), iron (Fe), and cobalt (Co). The catalytic metal 8 is deposited using sputter, evaporater or metal solution. As a high density direct current 10 is applied to the catalytic metal 8 for generating heat, the catalytic metal 8 acts as a catalyzer in crystallization of the amorphous silicon layer 4. In other words, silicide generated in the chemical reaction between the amorphous silicon layer 4 and the catalytic metal 8 acts as a seed of the crystallization.
In FIG. 1B, a power source 10 applies a high density direct current to the catalytic metal 8, thereby crystallizing the amorphous silicon layer 4 (shown in FIG. 1A) into a polysilicon layer 5. After the crystallization, the catalytic metal 8 is removed from the substrate 1, and the polysilicon layer 5 is patterned into a polysilicon island 12, as shown in FIG. 1C.
In FIG. 1D, a second insulating layer 14 referred to as a gate insulating layer and a gate electrode 16 are sequentially formed on the polysilicon island 12. Then, an ion doping is carried out injecting dopants into the polysilicon island 12 to form an active region 18, a source region 20, and a drain region 22 in the polysilicon island 12. The active region 18 is a pure silicon region, whereas the source and drain regions 20 and 22 are doped silicon regions. The active region 18 is disposed on the buffer layer between the source and drain regions 20 and 22, while the gate insulating layer 14 and the gate electrode 16 are positioned on the active region 18.
Because the gate insulating layer 14 and the gate electrode 16 are patterned with the same mask in order to reduce the number of masks, they have the same shape. When the ion doping is applied to the polysilicon island 12, the gate electrode 16 serves as an ion stopper to prevent the dopant from penetrating into the active region 18. After the ion doping is finished, the polysilicon island 12 implements a specific electric characteristic, which varies with types of the dopants. If the dopant is, for example, B2H6 that includes a Group III element, a doped portion of the polysilicon island 12 becomes a p-type semiconductor. Whereas, if the dopant is PH3 that includes a Group VI element, the doped portion of the polysilicon island 12 becomes an n-type semiconductor. A proper dopant should be selected to satisfy the use of a device. After the dopant is applied onto the polysilicon island 12, the dopant is activated.
In FIG. 1E, a third insulating layer 24 that serves as an interlayer insulating layer is formed to cover the gate electrode 16, the active region 18, and the source and drain regions 20 and 22. A source contact hole 20a and a drain contact hole 22b are formed in the third insulating layer 24, thereby exposing the source and the drain region 20 and 22, respectively.
In FIG. 1F, a source electrode 26 and a drain electrode 28 are formed on the third insulating layer 24. The source and the drain electrode 26 and 28 electrically contact the source and the drain region 20 and 22, respectively, through the source and the drain contact hole 20a and 22a. Thereafter, a passivation layer 30 and a pixel electrode 34 are sequentially formed to cover the source and the drain electrode 26 and 28. The passivation layer 30 has a pixel contact hole 32 therein. Thus, a portion of the drain electrode 28 is exposed, and the pixel electrode 34 electrically contacts the drain electrode 28 through the pixel contact hole 32.
In the conventional process of fabricating the polysilicon TFT, a small amount of the catalytic metal is deposited on or below the amorphous silicon layer. Thereafter, a high density direct current is applied to form a crystallized silicon layer from an amorphous silicon layer. However, the method to deposit the catalytic metal on the surface of the amorphous silicon layer has a problem that the catalytic metal is oxidized in the air. Thus, the amount of the catalytic metal has to be increased to avoid the oxidation problem. Nonetheless, it invites another problem that a grain size of the silicon crystal becomes very small.
In addition, in the method to deposit the catalytic metal below the amorphous silicon layer, a part of the catalytic metal is oxidized when the insulating layer is oxidized. Moreover, it is difficult to form crystalline silicon with a good quality.
Accordingly, the present invention is directed to a method for fabricating a polysilicon TFT that substantially obviates one or more of problems due to limitations and disadvantages of the related art.
Another object of the present invention is to provide a method for fabricating a polysilicon TFT in which a catalytic metal is added in the middle of an amorphous silicon layer resulting in a polysilicon layer with good electrical characteristics.
Additional features and advantages of the invention will be set forth in the description which follows and in part will be apparent from the description, or may be learned by practice of the invention. The objectives and other advantages of the invention will be realized and attained by the structure particularly pointed out in the written description and claims hereof as well as the appended drawings.
To achieve these and other advantages and in accordance with the purpose of the present invention, as embodied and broadly described, a method of fabricating a polysilicon TFT includes forming a buffer layer on a substrate, forming a first amorphous silicon layer on the first insulating layer, forming a plurality of metal clusters on the first amorphous silicon layer, forming a second amorphous silicon layer on the metal clusters including the first amorphous silicon layer, and simultaneously applying a heat-treatment and a voltage to crystallize the first and the second amorphous silicon layers.
The catalytic metal is selected from a group consisting of nickel (Ni), paladium (Pd), iron (Fe), and cobalt (Co).
The method of fabricating a polysilicon TFT according to the present invention includes forming a buffer layer on a substrate, forming a first amorphous silicon layer on the buffer layer, forming a plurality of metal clusters on the first amorphous silicon layer, forming a second amorphous silicon layer on the metal clusters including the first amorphous silicon layer, simultaneously applying a heat-treatment and an electrical field to crystallize the first and the second amorphous silicon layers, thereby forming first and second polysilicon layers, patterning the first polysilicon layer to form a polysilicon island, and forming a gate electrode, a source electrode, and a drain electrode.
It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory and are intended to provide further explanation of the invention as claimed.